A nitride-based semiconductor high-electron-mobility transistor (HEMT) has an epitaxial layer structure including a barrier layer (typically AlGaN) and a channel layer (typically GaN) that together generate a two-dimensional electron gas (2DEG) at their interface. The two-dimensional electron gas forms due to piezoelectric polarization electric fields at gallium nitride/aluminum gallium nitride (GaN/AlGaN) interfaces. The conductivity of the channel layer can be modulated by a gate contact on the barrier layer, which allows current to flow between source and drain contacts on the channel layer.
While nitride-based HEMTs are useful for microwave frequency applications, their usefulness for other applications is limited due to the fact that they are normally ON devices. A normally OFF nitride-based HEMT has been developed by providing a Schottky junction between the source contact and the channel. An example of a normally OFF nitride-based HEMT is shown in L. Yuan et al., “Normally Off AlGaN/GaN Metal-2DEG Tunnel-Junction Field-Effect Transistors,” IEEE Electron Device Letters, Vol. 32, p. 303 (March 2011).
A conventional structure of a normally OFF nitride-based tunnel junction HEMT is illustrated in FIG. 1. As shown therein, a normally OFF nitride-based tunnel junction HEMT 10 includes a substrate 12 on which a GaN channel layer 14 is formed. An AlGaN barrier layer 16 is on the GaN channel layer 12 and forms a two-dimensional electron gas 15 at the AlGaN/GaN heterojunction 17.
A reactive ion etching (RIE) mesa isolation using a chlorine/helium (Cl2/He) inductively coupled plasma etch process may be used to define a device mesa. An ohmic drain electrode 18 is deposited using electron-beam (e-beam) evaporation and a rapid thermal anneal.
A source electrode 20 is formed on the opposite side of the channel layer 14 from the drain electrode 18. To form the source electrode 20, the epitaxial structure is etched at least to the level of the 2DEG 15 using, for example, a further Cl2/He etch. The source electrode 20 is formed of a metal, such as titanium-gold, that forms a Schottky contact with the material of the channel layer 14. The source electrode 20 may be deposited using e-beam evaporation and lift-off. The source electrode may be self-aligned to the etched side of the 2DEG 15 by combining photolithography for etch and acetone lift-off steps.
The source electrode 20 forms a tunnel junction 30 with the 2DEG 15 in the channel layer 14. The Schottky barrier height of the contact is about 0.8 eV, while the Schottky barrier width is only a few nanometers. Thus, efficient quantum tunneling may occur at larger currents.
An insulating layer 22, which may include aluminum oxide, is formed on the barrier layer 16 and extending onto the source electrode 20 and drain electrode 18. A gate electrode 26 is then formed, for example by e-beam deposition and liftoff, on the insulating layer between the source 20 and drain 18 electrodes.
Source 24 and drain 28 pads are formed on the source 20 and drain 18 electrodes, respectively.
A HEMT as shown in FIG. 1 may be a normally-off device with a positive threshold voltage, which may reduce power consumption in some applications. The device may also exhibit reverse current blocking at the source-channel interface, which may suppress leakage through the buffer layer as compared to conventional HEMT structures.